Question

List at least five parasites. State their life cycles, and the methods by which they infect their hosts.

Answer 1

Helminths are the eukaryotic multicellular worms. They are the parasitic organisms, means acquire energy from the host cell. Their size can range from 0.3 millimeters to 25 meters long. The three groups of helminths include,

1. Roundworms (nematodes)
2. Flukes (trematodes)
3. Tapeworms (cestodes)

The four ways through which the helminths infect humans are classified as life cycles A-D.

• The life cycle A – includes the transmission of the parasite through the fecal-oral route
• The life cycle B – Includes the transmission of the parasite through the fecal-environment-skin route
• The life cycle C – Fecal-intermediate host-oral route
• The life cycle D – Combination of lifecycle A and B

All the helminths require an intermediate host to complete their lifecycles. The worms use more than one definitive host, and a few worms have intermediate hosts.

Parasite 4:

Schistosoma mansoni is a bisexual flatworm (class Trematoda). It belongs to the phylum Platyhelminthes and is and parasite that is located in the liver and intestine. The primary hosts of the organism are vertebrates and the secondary hosts are generally snails.

Mode of transmission: Through contaminated water

Life cycle of S. mansoni:

The organism undergoes sexual reproduction in the humans and asexual reproduction in snails. The eggs (miracidium) of the organism are released out through the human feces and the matured miracidia (larva) penetrates the snail.

In its sporocyst form, it grows in the snail (needs aquatic environment). The cercariae (infective larva) releases out of the snail and penetrates into the skin. From the skin, the organism escapes into the systemic circulation and infects other organs.

Parasite 5:

Malaria is a parasitic infection transmitted by the Anopheles mosquitoes, the causative organism of malaria is Plasmodium vivax, P. falciparum. P. ovale, P. malariae, or P. knowlesi.

Malaria mode of transmission: Through the bite of an infective female Anopheles mosquito, contaminated needles, and blood products

The life cycle of plasmodium:

1. Asexual phase - sporozoites enter a blood capillary
2. Exoerythrocytic phase - the sporozoites infect the liver cells and produce merozoites
3. Erythrocyte phase - the merozoites infect the erythrocytes and give rise to ring trophozoite
4. Gametophyte phase - the release of gametes by the infected erythrocytes, which infect a mosquito
5. Sexual phase - The gametes enter into a mosquito and the organism develop completely into an infectious sporozoite.